LONDON, Dec. 13 (Xinhua) -- Another 18,447 people in Britain have tested positive for COVID-19, bringing the total number of coronavirus cases in the country to 1,849,403, according to official figures released Sunday.

The coronavirus-related deaths in Britain rose by 144 to 64,170, the data showed.

Earlier Sunday, NHS (National Health Service) Providers, which represents health trusts in England, urged British Prime Minister Boris Johnson in a letter to exercise "extreme caution" when reviewing the coronavirus tiers system.

NHS chiefs said that areas which need to be moved into the highest Tier 3 should be done so "as soon as this is needed, without any delay".

Health workers are "really worried" as hospitals are facing a difficult period due to a surge in COVID-19 patients, NHS Providers CEO Chris Hopson told Sky News.

"Any extra social contact equals virus spread," Hopson said.

His remarks came ahead of the relaxation of restriction rules for Christmas, between Dec. 23 to 27, allowing up to three households to get together in homes and places of worship.

"We all want to get together, but we need to ensure we look at the evidence," he said. "Although we've made some good progress... the reality is the progress we've made as a whole is at the lowest end of expectations."

"We've got to keep going with these tough restrictions over winter," he said. "We cannot take risks."

England is currently under a new three-tier system of coronavirus restrictions that replaced the month-long national lockdown in England which ended on Dec. 2.

The tiers system is due to be reviewed on Wednesday.

Under the current tiers system, shops are allowed to re-open across England, giving a Christmas bonanza to the struggling retail sector, but all bars and restaurants in the toughest Tier 3 areas remain closed. The system put about 98 percent of England into the highest Tier 2 and 3.

To bring life back to normal, countries such as Britain, China, Germany, Russia and the United States are racing against time to develop coronavirus vaccines.